The present invention relates to a method of producing a pulse width modulated signal, and more particularly to a method of producing such signal using only digital techniques and digital apparatus.
Pulse width modulation of an electrical signal is a common method of encoding information. This type of modulated signal is often used to drive some sort of electromotive power unit, such as a servo-motor. A typical system might employ a servo-motor, driven by such signal, to control; a manufacturing process, the flight of an airplane, or some other real-time event. This type of servo-control is extensively employed in many drone environments; i.e., those in which no operator is actually present, such as guided missiles or the like. An early pulse width modulation system used on guided missiles simply combined an analog ramp wave with an analog command signal received from an autopilot. With the advent of more sophisticated electronic apparatus and the general trend toward digitalization, this analog method became less and less compatible with the rest of the overall system. However, totally digital systems have proved less than perfect, with such imperfections evidenced by a lack of smooth airframe response, resolution deficiencies and inexactitudes in the performance of command signals.
To further show some of the problems presented by a totally digital system, a prior hybrid (analog and digital) system might be examined. To pulse width modulate a command signal, say, a digital one at some frequency f.sub.m, this command signal may be compared to a modulation reference wave having a typical period of 1/f.sub.m. A typical system employs an analog ramp wave as the modulation reference and a comparator to indicate when both waves are of equal value, i.e., when one wave crosses the other. Each time the comparator detects such crossing a binary command may be sent to an on-off actuator connected to the particular servomechanism under control. No resultant command signal errors will be introduced whether the initial command signal is in analog or digital form, provided the modulation reference is an analog signal. However, when the modulation reference wave is in digital form and the command signal is in digital form, resolution problems arise. A typical digital modulation reference signal might consist of a series of discrete steps of increasing amplitude arranged to approximate an analog ramp wave. In other words, instead of the modulation reference being a smooth wave it is more like a staircase wave. In this case, when the command signal intercepts the reference wave and the comparator indicates that the waves are of equal value, the indicated actual point of intercept will lead or lag (in time) the ideal intercept point obtained by use of an analog ramp wave.
Therefore, it is an object of the invention to provide a digital method of pulse width modulation.
It is a further object of the invention to provide a digital pulse width modulation method having an effective resolution which is not primarily determined by the ratio of computational frequency to modulation frequency.
It is a still further object of the invention to provide a digital pulse width modulation method capable of being performed on a general purpose digital computer.